A passive matrix liquid crystal display ("LCD") is one popular type of display whose display elements respond to the values of the rms voltages applied across them. A passive matrix LCD includes overlapping row electrodes and column electrodes positioned on opposite sides of a film of liquid crystal material. The locations where the row and column electrodes overlap define the display elements. The portion of liquid crystal film associated with each display element is an electro-optic material that responds to a change in the value of an rms voltage applied across the display element to provide a corresponding change in the amount of light passing through it. The liquid crystal device most prevalently used in such displays is of a supertwisted nematic ("STN") type. The row electrodes receive addressing signals that select the rows at various times, and the column electrodes receive data signals that represent the information patterns to be displayed.
There are optimum values of the rms voltages that can be applied across the display elements to provide light transmitting ("ON") and light blocking ("OFF") optical states. For standard addressing, the rows are sequentially selected typically with a 21-volt pulse, and the display system requires a 16.7 millisecond repetition period to select all of the rows one at a time. These row addressing waveforms, which are called Alt and Pleshko waveforms, are normally used to provide the optimum voltages, but they are of highly nonuniform amplitude as a function of time.
If it can respond within the repetition period of the row select waveforms, an LCD will not respond to the true rms value of the waveforms. This phenomenon is called a "frame response" effect. The frame response causes the transmission characteristic of the LCD to be different from that which is intended. For example, a display element driven to remain in the OFF optical state in successive frames will leak light during a portion of the frame period. The result is a lower contrast ratio for the display element. If an LCD is constructed to respond sufficiently fast so that its display elements switch at video rates, the frame response effect can become so severe that it is difficult to make a passive matrix LCD that is capable of displaying video rate images.
Two approaches have been used to solve the frame response effect problem. One approach is to increase the frame rate of the Alt and Pleshko waveforms, and the other approach is use waveforms whose amplitudes are more uniform with time.
The first approach has two difficulties. One difficulty is that the row and column signal drivers need to operate at higher than normal rates, thereby increasing circuit complexity and power consumption. The second difficulty is that signals of higher frequencies are applied to the row and column electrodes of the display. Because the row electrodes are of high resistance and the row signals drive significant capacitance, the use of higher frequency signals makes it difficult to maintain the desired voltage levels across the display elements down the rows and columns of the display. The second approach, which is sometimes called "active addressing," requires more complex drive circuit electronics.
STN displays are often coupled with compensator cells or sheets of compensator material to improve the color quality (i.e., by making a black-and-white display instead of a blue-and-white or a yellow-and-black display) and improve the viewing angle performance. An electrically driven compensator cell includes a second STN liquid crystal cell of opposite liquid crystal molecular helical twist direction that offsets the birefringence resulting from color dispersion effects of the STN display cell. The compensator cell does not usually contain patterned row or column electrodes.